This invention relates to a matrix display apparatus for displaying video signals of two or more different standards which apparatus comprises a display panel having an array of picture elements arranged in a set number of rows and columns and driving means for driving the picture elements according to an applied video signal, the driving means comprising a column driver circuit operable according to the line frequency of an applied video signal for supplying data signals derived therefrom to the columns of picture elements, a row driver circuit for scanning the rows of picture elements in turn, and a control circuit which controls the timing of the operation of the row driver circuit and which is responsive to an applied video signal to control the row scanning rate of the row driver circuit according to the standard of the video signal. The invention relates also to a method of operating such a display apparatus. The invention is concerned particularly, although not exclusively, with liquid crystal display apparatus for displaying TV pictures.
Normally, a matrix liquid crystal display panel is designed for use with one video standard with the number of rows of picture elements being selected having regard to the number of display lines of the video standard. However this leads to problems if it is desired to use the display panel to display video signals of a different standard, as might be required, for example, for a portable TV receiver. If an NTSC (525 line) video signal is displayed on a panel designed for PAL (625 line) video signals a reduced height picture having the wrong aspect ratio is obtained. An external standards-converter could be employed to convert for example an NTSC signal to the PAL standard but this involves complex signal processing techniques and is very expensive in terms of the required circuitry.
There is described in U.S. Pat. No. 5,003,388 a scheme for enabling, for example, a PAL standard video signal to be displayed on a display panel designed for an NTSC standard video signal. In this scheme, converter means are employed which change the number of display lines of the video signal that are actually displayed by discarding selected display lines, in this case every sixth line of the PAL standard signal, thereby enabling a picture according to the PAL standard to be displayed on the display panel. In the apparatus described, a row and column array of picture elements, for example liquid crystal elements, is driven by a data driver circuit which supplies data signals derived from a video signal to the columns of picture elements and a scan driver circuit connected to the rows of picture elements which scans each row in turn. The operations of the data and scan driver circuits are controlled and synchronised by a control circuit using synchronising signals derived from the video signal. The control circuit operates the scan driver circuit at a rate corresponding to the line frequency of the video signal. When, for example, a PAL signal is to be displayed, the scan and data driver circuits are controlled so as to ignore selected, regularly spaced, lines in the video signal, and the sampled data signals pertaining thereto. In this way the PAL standard picture can be made to fit the picture element array. However, as described in this specification, unless certain measures are taken unwanted display artefacts can be produced using this approach. Even if the described measures are taken, which involve changing the position of the deleted lines from field to field, it is likely that display disturbances such as flickering lines will be observed when displaying certain kinds of pictures.
To display an NTSC video signal on a display panel designed for PAL video signals an opposite approach may be possible whereby certain lines in the NTSC signal may be repeated so that in places several rows of picture elements are driven with the same picture information. In this way the picture is effectively expanded in the vertical direction to fill the available display area. Again however, this technique may lead to perceivable display artifacts affecting the quality of the displayed picture. When lines in the picture have to be repeated there is then less time available to address the rows of the display panel and this can result in differences in the picture element voltages for repeated and non-repeated lines.